With advances in electronic products, semiconductor technology has been applied widely manufacturing memories, central processing units (CPUs), liquid crystal displays (LCDs), light emitting diodes (LEDs), laser diodes and other devices or chip sets. In order to achieve high-integration and high-speed requirements, dimensions of semiconductor integrated circuits have been reduced and integrated circuits including various (“mixed”) supply voltages (Vdd) have been proposed.
FIG. 1 shows an example of a system having a driving device 110 operating at first supply voltage (Vdd=1.8 Volts), and having an output, and a driven device 150 operating at a second supply Voltage (Vdd=1.2 Volts) lower than the first supply voltage. For example, devices 110 and 150 may be phased look loops (PLL), ring oscillators, logic blocks, and/or other circuits, and/or combinations thereof. Both devices have a common lower-voltage power supply rail Vss. The driving device 110 has a CMOS output buffer, including a PMOS pull-up transistor 112 and an NMOS pull-down transistor 114. The driven device 150 has a CMOS input buffer, including a PMOS pull-up transistor 152 and an NMOS pull-down transistor 154.
The-output 116 of driving device 110 is provided as a control signal to the gates of the CMOS input buffer of the driven device 150. However, the gate oxide of a metal-oxide-semiconductor (MOS) transistor of an integrated circuit is most susceptible to damage.
When an ESD event occurs, an ESD voltage is conducted by the higher-voltage power supply rail Vdd of the driving device to the PMOS transistor 112 of the CMOS output buffer. The drain of the PMOS transistor 112 is connected to the output terminal 116 of driving device 110, which is connected by an interface to the input 156 of driven device 150. CMOS input 156 is connected to the gate of the NMOS pull-down transistor 154 of the CMOS input buffer. The gate oxide of NMOS 154 may be destroyed by being contacted with a voltage only a few volts higher than the supply voltage. Electrostatic voltages from common environmental sources can easily reach thousands, or even tens of thousands of volts.
Such voltages are destructive even though the charge and any resulting current are extremely small.